1. Field of the Invention
The present invention relates to an alignment sheet, an image forming apparatus and a method for aligning an image forming position.
2. Description of Related Art
Image forming apparatuses having a duplex image forming function of forming images on both sides of a sheet that are in widespread use include printers, copiers and multi-function peripherals.
Regarding duplex image forming, it is generally known that even when the apparatus is controlled so that the image forming positions on the front and back sides of a sheet are aligned with each other, misalignment between the image forming position on the front and back sides occurs in practice due to sheet shrinkage that occurs in the developing, transferring and fixing steps of the image forming on the first side.
To cope with the problem, the image forming position is normally corrected by using a preset correction value when an image is formed on the second side.
A method for calculating a correction value has been proposed, which involves providing a sheet (reference chart) with reference marks formed at the four corners of both sides, reading both sides of the reference chart with an image reading unit to calculate the distance from an edge to each mark on both sides, and determining the correction value from the difference between the calculated distances (for example, see JP 4438943B).
However, in the method described in JP 4438943B, a sheet (assist member) that has a different brightness and a larger size to that of the reference chart is placed on the back side of the reference chart while reading the reference chart so that the edges of the reference chart are emphasized and that the distance from an edge to each mark is precisely calculated.
Therefore, the calculation of a correction value requires the assist member in addition to the reference chart, which makes the process complicated.